The present invention relates to a method and an apparatus for speed change control of an automatic transmission for motor vehicles.
Automatic transmissions installed in motor vehicles comprise a large number of frictional engaging elements, such as hydraulic multiple disc clutches, hydraulic brakes and the like. Among these clutches and brakes, those to be operated are subjected to switching control by a controller, to carry out a shift of the automatic transmission.
The hydraulic multiple disc clutches and the hydraulic brakes each include a large number of friction plates. In such clutches and brakes, a predetermined clearance is provided between adjacent friction plates, taking machining error and assembling error into consideration, to prevent drag torque from occurring between the friction plates when the clutch or brake has been released.
Accordingly, when engaging the clutch or the like which is completely disengaged, the piston must be moved so at to make the aforementioned clearance zero, or a so-called dead space (clearance) elimination must be carried out, after the application of hydraulic pressure is started. Thus, a predetermined dead space elimination time (hydraulic pressure supply time) is required before the friction plates actually become engaged with each other.
On the other hand, to fully disengage the clutch or the like, after the supply of hydraulic pressure has been stopped, the friction plates; must be separated from each other to make the aforementioned clearance therebetween, while the operating oil in the clutch is discharged by the force of a return spring or the like. Thus, a predetermined hydraulic pressure release time is required before the transmission torque is reduced to zero.
In the automatic transmission described above, when downshift from the second to the first speed, for example, is effected, the controller disengages a second-speed clutch for establishing the second speed and engages a first-speed clutch for establishing the first speed, in accordance with a predetermined program. This increases the rotational speed Nt of a transmission input shaft such that a change rate (Nt)' thereof becomes equal to a target change rate (Ni)'. Thereby, changeover of the clutch connection can be carried out. The controller then estimates the time at which the rotational speed Nt of the input shaft becomes synchronized with a first-speed synchronous rotation speed, and starts driving a solenoid valve for controlling the first-speed clutch at a 100% duty factor at the point of time preceding the estimated time by a predetermined period. Thus full hydraulic pressure is applied to the first-speed clutch. As a result, engagement of the first-clutch rapidly progresses and is completed at the estimated time. Accordingly, the engine torque transmission path is switched from the second-speed clutch to the first-speed clutch, and the automatic transmission downshifts from the second speed to the first speed.
When the accelerator pedal at the driver's seat is depressed during downshift and thus the engine is in a power-on state, the rotational speed Nt of the input shaft is going to rise. On the other hand, when the accelerator pedal is released during downshift and the engine is in a power-off state, the rotational speed Nt of the input shaft does not increase.
To cope with such difference, the controller prestores therein a control program for carrying out downshift during power-on state and a control program for carrying out downshift during power-off state. When a downshift is to be effected, the controller first detects the engine drive state, and selects one of the control programs in accordance with the result of the detection.
More specifically, when it is determined that the engine is driven in a power-on state, the controller executes the power-on program. In accordance with this power-off program, which the second-speed clutch is gradually disengaged to suppress a sudden increase of the rotational speed Nt of the input shaft, and the input shaft speed Nt is increased such that the change rate (Nt)' thereof coincides with the target change rate (Ni)'. The controller then actuates the first-speed clutch at predetermined timing such that the first-speed clutch becomes completely engaged when the input shaft speed Nt reaches the first-speed synchronous rotation speed, thereby carrying out the downshift.
On the other hand, when it is determined that the engine is driven in a power-off state, the controller executes the power-off program. In accordance with the power-off program, the second-speed clutch is immediately disengaged, and then the first-speed clutch is gradually engaged to increase the rotational speed Nt of the input shaft such that the change rate (Nt)' thereof becomes equal to the target change rate (Ni)'. After the rotational speed Nt reaches the first-speed synchronous rotation speed, the controller completely engages the first-speed clutch, thereby effecting the downshift.
If, during a downshift, the driver depresses the accelerator pedal and the engine drive state changes from a power-off state to a power-on state or vice versa, or if the controller makes an erroneous determination when the engine is operating in a boundary region between the power-on and power-off states, the controller executes an improper program that does not match the engine drive state, causing the following disadvantages.
Namely, if the controller determines that the engine is in a power-off state and executes the power-off program, although the engine is actually in a power-on state, the input shaft, of which the rotational speed is rapidly increasing, is suddenly coupled to the first-speed clutch, causing a large shift shock. If, on the other hand, the controller determines that the engine is in a power-on state and executes the power-on program, although the engine is actually in a power-off state, the rotational speed of the input shaft does not increase, thus causing problems such that the progress of shift operation is hindered.
Further, as mentioned above, an estimate is made of the time at which the rotational speed Nt of the input shaft becomes synchronized with the first-speed synchronous rotation speed, and the first-speed clutch is quickly engaged based upon the estimated time. Therefore, if the accelerator pedal is operated during execution of a downshift and the estimated time becomes improper, for example, the input shaft, which is not synchronized, is abruptly coupled to the output shaft, causing problems suck as a large shift shock.
Furthermore, during the changeover of clutch connection, if engagement of the first-speed clutch is started when the second-speed clutch is not yet disengaged, interlocking (both the first-speed clutch and the second-speed clutch are engaged) occurs, possibly locking the transmission. Conversely, if the first-speed clutch is engaged too late, failure of clutch connection (neither the first-speed clutch nor the second-speed clutch is engaged) occurs, causing an undesired rise or drop of the engine rotation speed, depending upon the operating states, and prolonging the shift time.